As well known in the art, the process for producing kraft pulp basically comprises: (A) a cooking step of treating raw chips with a cooking liquor containing sodium hydroxide and sodium sulfide as main components to convert the chips into pulp; (B) a pulp washing step of washing the resultant pulp, and separating and recovering a black solution containing sodium carbonate and sodium sulfate, which are turned from the cooking liquor; (C) a pulp bleaching step of treating the pulp with a bleaching agent in the presence of alkali; (D) a black solution concentrating step of concentrating the black solution separated and recovered in the pulp washing step (B); (E) a black solution combustion step of burning the concentrated black solution to reduce sodium sulfate into sodium sulfide and further recovering sodium sulfate and sodium carbonate from a combustion exhaust gas by a dust collector; and (F) a causticization step of treating a green solution as an aqueous solution of a smelted product recovered in the combustion step with calcium oxide to convert sodium carbonate contained in the green solution into sodium hydroxide, thereby obtaining a white solution, wherein the white solution recovered in the causticization step (F) is recycled to the cooking step (A).
In the above-described process for producing kraft pulp, chemicals used therein have been recovered in a closed system to enhance a recovery percentage thereof more and more. More specifically, sodium carbonate or sodium sulfate has also been recovered from ashes captured and recovered from a combustion exhaust gas generated in the combustion step (E) by the dust collector.
Meanwhile, in the case where valuable substances are recovered from the captured ashes as described above, it is required to remove impurities turned from raw wood materials, for example, potassium components in order to prevent accumulation of these impurities in the recovered valuable substances.
As a method of removing common salt (sodium. chloride) and potassium salts from ashes captured in a sodium-recovering boiler, there has been proposed the method of adjusting a pH value of a water slurry containing the captured ashes to not more than 10 by adding sulfuric acid thereto and further adjusting a temperature of the water slurry to not less than 20° C.; keeping the water slurry under the above condition for a predetermined period of time to dissolve common salt and potassium salts contained in the captured ashes in water; cooling the resultant water slurry to a temperature less than 20° C. to precipitate solids; re-dissolving the solids in the black solution before being concentrated; and returning the resultant black solution to an upstream side of a concentrator for the black solution, thereby recovering the solids (Japanese Patent Application Laid-Open (KOKAI) No. 9-29201(1997)).
However, the above conventional method has the following problems: (1) a loss of valuable substances is large since a large amount of these substances are contained in an effluent discharged out of the system; (2) a large amount of dilute sulfuric acid should be used for enhancing a sodium recovery percentage; (3) increased costs are required for operating an ice maker for producing ice used in a precipitation tank; (4) running costs are disadvantageously large owing to these problems; and (5) equipments used tend to suffer from abrasion and deterioration due to the slurry.
Under these circumstances, in order to overcome the above problems, the present inventors have proposed as the method of treating ashes captured in a cooking chemical recovery step, such a method of treating an aqueous solution of the captured ashes with an ion exchange resin to recover and reuse a fraction containing a large amount of sulfate ions and carbonate ions (Japanese Patent Application Laid-Open (KOKAI) Nos. 2002-138381, 2002-138382 and 2002-146691).
In the above Japanese Patent Applications, there are described a step of adsorbing and removing potassium ions using Na-type strong acid ion exchange resins, a step of adsorbing and removing polyvalent metal ions using chelate resins, and a step of separating chlorine ions using ampholytic ion exchange resins. Further, in these Japanese Patent Applications, as a method of regenerating the Na-type strong acid ion exchange resins and chelate resins which need for regeneration of chemicals, there are described the method of flowing an acid (for example, an aqueous hydrochloric acid solution) through the resins and then flowing an aqueous sodium hydroxide solution or an aqueous sodium chloride solution through the resultant resins, and the method of flowing an acid (for example, an aqueous hydrochloric acid solution) through the resins and then flowing an aqueous sodium hydroxide solution through the resultant resins.
An object of the present invention is to provide an industrially useful process for producing kraft pulp, which is capable of recovering chemicals in a closed system, in particular, preventing, accumulation of potassium impurities, and effectively utilizing the chemicals used in the process.